In oil production, it is common for the extracted oil to contain a large proportion of water which is also present in the reservoir. In addition, some oil production techniques include injecting steam into the oil reservoir to permit or enhance recovery of oil from the reservoir. After the oil has been separated from the water, the remaining water is still contaminated. Advantageously, in oil injection methods the contaminated water is treated and then recycled to a steam generator for use in subsequent oil extraction. This of course reduces the amount of water needed for the extraction process.
Even though such water recycling can be extremely efficient, some additional water is required in the process to make up for losses. This is referred to as makeup water. Conventionally, this makeup water is fresh water with relatively low salinity and hardness.
In some circumstances, it may be desirable or even legally required to use brackish water to provide at least a part of the makeup water. Brackish water has a high salinity and hardness. In known water recycling plants, the water treatment is carried out using an evaporator with a relatively high pH, to maintain silica in solution to reduce scaling. Introducing brackish water with high salinity and alkalinity into the high pH environment causes problems with substantial deposition within the evaporator, causing scaling, requiring periodic shutdown and cleaning to maintain the efficiency of the evaporator.
It is known to deal with this problem by using a pre-treatment step including a cold lime softening (CLS) stage to reduce the hardness of the brackish makeup water. This is, however, unsatisfactory in that the hardness levels are still high after pre-treatment, additional equipment, chemicals and energy are required to operate the system and additional sludge is generated which requires safe disposal.